1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly to an injection molding machine that determines whether a check ring is closed or not during forward movement of a screw and rotates the screw in reverse by an amount necessary to close the check ring.
2. Description of the Related Art
In a known technique described in, for example, Japanese Patent Application Laid-Open No. 62-060621, the check ring in an injection molding machine is closed by rotating the screw in reverse (or in the direction reverse to the direction of the screw during metering process) at the start of injection. The amount of reverse rotation of the screw is calculated and set according to the spacing between the check ring and check seat when the check ring is open.
In another known technique described in, for example, Japanese Patent Application Laid-Open No. 1-308611, the screw is rotated in reverse in synchronization with the injection speed during injection process. In yet another known technique described in, for example, Japanese Patent Application Laid-Open No. 60-76321, the check ring is closed by rotating the screw in reverse while moving it in positive direction during the period between metering and injection.
The amount by which the screw must be rotated in reverse to close the check ring varies with the resin viscosity, injection speed, and other conditions. Therefore, if the amount of reverse rotation at injection is set as described in the above conventional art, the amount may be not enough to close the check ring completely, or the amount may be excessive, depending on the resin and molding conditions.
If the amount of reverse rotation of the screw is not enough to close the check ring completely, the amount of resin injected becomes disadvantageously unstable. If the amount of reverse rotation is excessive, an extra amount of resin is fed behind the screw, causing problems of increased metering time and accumulation of resin.